Controlling the spectral characteristics of an optical source of radiation is used in various aspects of science and engineering. For instance, regularly spaced optical spectral lines, commonly called optical combs, can be used for highly precise metrology. Sources where the optical frequency is swept or “chirped” in time are used for biological imaging as well as optical ranging applications. The invention herein is a source of optical radiation whose spectrum can be tuned or reconfigured to generate many different spectral profiles, including an optical spectral output that varies in time. It makes use of a nonlinear optical interaction, such as the parametric gain induced by four-wave mixing in optical fiber, in order to control the spectral output of the source.
A scheme for realizing an optical source using a gain medium and a frequency shifting element configured in a recirculating loop has been disclosed in U.S. Pat. No. 5,101,291 and is often termed a frequency shifted feedback laser (FSFL) because the propagating mode is shifted in frequency before being fed-back to the gain element. This source uses an acousto-optical modulator to generate the frequency shift. Such a source has been used in various modes of operation including generating a standard fixed optical comb or a chirped optical comb {L. P. Yatsenko et al., “Theory of a frequency-shifted feedback laser,” Optics Communications 236, 183-202 (2004)}, and generating an output with a narrow-band optical frequency that varies in time {H. Takesue and T. Horiguchi, “Broad-Band Lightwave Synthesized Frequency Sweeper Using Synchronous Filtering,” J. Lightwave Technol. 22, pp. 755-757, 2004}. Although the output frequency varies in time, it does so by making a simple fixed frequency shift every round trip. The frequency shift is induced by an acousto-optic Bragg cell, which has a limited range of frequency shifts. This makes it difficult to tune the operation of the FSFL over a broad range of conditions including making frequency shifts that can vary substantially over short time periods.
An extension of such work is described in U.S. Pat. No. 5,734,493 where other methods, such as a single sideband modulator, are used as the shifting method. In principle this allows a wider range of frequency spacing between adjacent modes of the device, however it is still limited by the electrical bandwidth of the modulator.
A fundamentally different type of frequency shift which exploits nonlinear optical effects is described in U.S. Pat. No. 6,856,450 B2. Here two or more “frequency mirrors” are used to perform the shifting. The frequency mirrors use nonlinear optical interactions that can have very broad bandwidths. The frequency shift is typically equal to twice or four times the difference in the optical frequency of two pump sources, depending on if the nonlinear interaction is a second or third order effect. This method allows for a much greater range of frequency shifts. Although U.S. Pat. No. 6,856,450 describes some aspects of the basic geometry, the application is narrowly focused on the generation of regularly spaced comb lines or of frequency shifting a modulated signal onto a regularly spaced grid. Also, the optical amplifier used in the system, which is optional and is used to compensate for loop losses if the nonlinear interaction is not strong enough to do so itself, needs to have an optical bandwidth equal to the generated signal bandwidth. Thus, in practice the optical amplifier can be the limiting factor in determining the system's optical bandwidth. The designs disclosed are also not well suited for the use of fiber-based nonlinear interactions. Fiber-based nonlinear interactions are desirable since it is simple to splice the nonlinear fiber to other fiber-coupled components to make a robust and easily manufacturable system. In the case of the third-order nonlinearity of fibers, special consideration is required to combine the pump and signal together, since they are of similar optical frequency. Direct couplers, such as a 50/50 splitter, experience an inherent insertion loss when combining signals. Also, fiber nonlinearities can suffer from stimulated Brillouin scattering (SBS), which can limit the effective pump power attainable and therefore the effective gain or bandwidth of the nonlinear interaction. Suppression of SBS has been addressed in the context of other fiber nonlinear systems such as standard parametric amplifiers {“Fiber Optical Parametric Amplifiers, Oscillators and Related Devices” by M. E. Marhic, Cambridge University Press 2008}, although the typical techniques cannot be directly applied to the frequency mirror system since without special precautions the frequency modulation of the pumps is transferred to the generated signal.
Substantial optical chirp can be generated using a single-sideband modulator driven by a chirped electrical frequency. However the amount of chirp is ultimately limited by the bandwidth of the modulator, unless very difficult optical techniques are used to try to increase the chirp {K. W. Holman et al, “MIT/LL development of broadband linear frequency chirp for high resolution ladar,” Proc SPIE v 6572 65720J-1, 2007}. Currently such optical techniques are not practical and are very difficult to scale to increase the chirp much past what is possible using a modulator alone. It is desired to be able to generate chirp over a larger frequency range.
Carrier suppressed return-to-zero (CSRZ) modulation can be used for generating a number of precisely controllable optical tones {C Yu, et al., “Multi-channel high-speed optical pulse train generation based on phase modulation at half frequency,” Lasers and Electro-Optics, paper CMJJ7, CLEO 2007}. The CSRZ modulation suppresses the input carrier and generates tones spaced at ±nf, where n is an integer and f is the modulating frequency. However, typically only a few tones of useful magnitude are generated with a CSRZ modulator. One interesting property of CSRZ modulation is that if the modulation signal to the CSRZ modulator is chirped, it will generate side bands on either side of the suppressed carrier that are chirped in opposite directions.
The use of two (or more) optical comb lines with different frequency spacing has been used previously for metrology applications {F. Keilmann et al, “Time-domain mid-infrared frequency-comb spectrometer,” Optics Letters v. 27, pp 1542-1544, July 2004}. However, they used two separate frequency locked optical comb systems. This is an expensive and bulky method of generating two combs or different spacing. A more compact and less expensive method is desired.
What is needed is a flexible method of controlling generated optical spectrum. For instance, a method of generating various types of chirped optical frequencies, including very large and fast optical chirps and optical combs that are uniformly chirped or have a chirped grid spacing. Chirp values larger or faster than those possible with electro-optical modulation alone are particularly useful. Another needed function is to generate optical frequencies that can be quickly reprogrammed to shift to another value where the magnitude of the shift can be programmed quickly and over a large range. Another need is for the generation of multiple combs each with a selectable comb spacing using a single robust device. These types of functions should be realized using optical techniques which are practical to implement. In many cases it is desirable to implement such an optical spectrum generator using fiber-based nonlinearities. In such cases, SBS may need to be suppressed and the pump and recirculating signal need to be combined with low losses. If an optical amplifier is required, it should be configured so as not to limit the optical bandwidth of the system.